Helical nuclear spin order in two-subband quantum wires

In quantum wires, the hyperfine coupling between conduction electrons and nuclear spins can lead to a (partial) ordering of both of them at low temperatures. By an interaction-enhanced mechanism, the nuclear spin order, caused by Ruderman-Kittel-Kasuya-Yosida exchange, acts back onto the electrons and gaps out part of their spectrum. In wires with two subbands characterized by distinct Fermi momenta k_F1 and k_F2, the nuclear spins form a superposition of two helices with pitches π/k_F1 and π/k_F2, thus exhibiting a beating pattern. This order results in a reduction of the electronic conductance in two steps upon lowering the temperature.